CHAP, i.] TISSUES AND MECHANISMS OF DIGESTION. 457 



and vice versa; and very possibly a like see-saw of stimulation and 

 inhibition obtains in the muscles of the alimentary canal. 



It must be remembered that the circular coat is always much 

 thicker than the longitudinal coat ; and we may infer that while 

 the chief work of driving the contents onward falls on the former 

 the latter assists the work, either in the way which we have 

 suggested or in some other way. 



In the small intestine the tube is hung loosely and much 

 twisted so that many loops are formed ; the contents moreover are 

 largely fluid. Hence the steady onward movement, such as is seen 

 when more solid contents pass along the straight and somewhat 

 firmly attached oesophagus, is complicated by movements due to a 

 loop being projected forward by the entrance of fluid from above, or 

 being dragged down by the weight of its new contents, or, on the 

 other hand, due to a loop being retracted by the driving onward of 

 its contents and the emptying of itself, and the like. In this way 

 a peculiar writhing movement of the bowel is brought about, and 

 the phrase ' peristaltic movement ' is generally used to denote this 

 total effect of the contraction of the muscular coats ; it will 

 however be best to restrict the meaning to the progressive 

 contraction of the circular coat assisted, in most cases, by a similar 

 progressive contraction of the longitudinal coat. 



270. Movements of the (Esophagus, These as we have just 

 said are fairly simple. The circular contraction begun by the 

 constrictors of the pharynx is continued along the circular coat of 

 the oesophagus and assisted by an accompanying contraction of the 

 longitudinal coat, the direction being always, save in the abnormal 

 action of vomiting, from above downwards. 



It will be remembered ( 222) that the muscular bundles of 

 the oesophagus are composed of striated fibres in the upper part, 

 and of plain unstriated fibre-cells in the lower part, the transition 

 occupying a different level in different animals. Nevertheless, as 

 far as the peristaltic movement is concerned, the two kinds of 

 fibres behave in the same way except that the peristaltic wave if 

 we may so call it travels more rapidly in the striated region. 



These peristaltic movements of the oesophagus may, like those 

 of the intestine, be seen after removal of the organ from the body ; 

 and indeed may continue to appear upon stimulation, for an 

 unusual length of time. They may therefore be carried out by 

 the muscular elements, with or without the help of the nervous 

 elements embedded in them, apart from any action of the central 

 nervous system. Nevertheless, in the living body, the move- 

 ments of the oesophagus seem to be in a special way dependent 

 on the central nervous system ; the contractions are not started 

 and carried out by the walls of the tube alone and so transmitted 

 from section to section in the walls of the tube itself ; but afferent 

 impulses started in the pharynx and passing to the medulla 

 oblongata, give rise to reflex efferent impulses which descend along 



